Light produced from an object existing in nature may have characteristic values in wavelength or similar unit. An image sensor is an apparatus that may pick up an image of an object by using the properties of a semiconductor device responsive to external energy. A pixel of an image sensor may detect light produced from an object and may convert it into an electrical value.
Such an image sensor may be classified into a charge coupled device (CCD) based on silicon semiconductor and a complementary metal oxide semiconductor (CMOS) image sensor fabricated by a submicron CMOS fabrication technology.
Of these image sensors, the CCD is a device in which charge carriers may be stored in a capacitor and transferred such that each MOS capacitor is closely disposed to each other. However, the CCD has various disadvantages, such as relatively complicated drive mode, relatively higher power consumption, impracticability of integrating a signal processing circuit in a chip for the CCD due to many mask processes and other reasons. In order to overcome these disadvantages, many studies may have been done towards development of the CMOS image sensor.
The CMOS image sensor may obtain an image by forming a photodiode (PD) and a MOS transistor within a unit pixel to detect signals in a switching mode. The CMOS image sensor may have the advantages of relatively low manufacturing costs, relatively low power consumption, and relatively easy integration into a peripheral circuit chip in comparison with a CCD. Since a CMOS image sensor may be produced using a CMOS fabrication technology, the CMOS image sensor may be easily integrated into a peripheral system for performing operations such as amplification and signal processing, resulting in minimized manufacturing costs. A CMOS image sensor may have a relatively rapid processing speed and a relatively low power consumption which corresponds to approximately 1% of the power consumption of the CCD.
Meanwhile, because a unit pixel of the CMOS image sensor may realize only one color, filters may be used for each pixel to filter only light of a desired wavelength from white light, and then red, green, and blue (RGB) values for each pixel may be calculated and restored by interpolation or similar. A color filter array for each unit pixel may be formed to realize red, green, and blue colors.
However, image sensors that may realize a fine line width circuit may be accomplished along with the development of semiconductor processing techniques. As the overall chip size, as well as the size of a unit pixel, gets progressively smaller with development, the size of each color filter is also minimized.
A material for producing a color filter array may be a polymer-based material, which may be very difficult to handle in an actual process and has a relatively high likelihood to undesirably maximize the rate of defects of manufactured devices. The polymer-based material of the color filter array may play a major role in minimizing the overall chip performance because it also serves to block the majority of light entering each unit pixel. This is because a color filter is supposed to selectively pass only light in a specific wavelength range, but may be unable to completely filter the light due to the characteristics of the color filter.
Therefore, there may be rise in demand for a method for realizing a color without the use of color filters.
FIG. 1 is a cross-sectional view of a color image sensor from which color filters are removed, according to the related art. Referring to FIG. 1, the image sensor according to the related art may include at least one of: (1) A substrate SUB which may be etched at a First thickness d1 in a first unit pixel region ‘a’ for realizing a color (e.g. R or magenta) of a first wavelength, etched at a second thickness d2 which may be relatively greater than the first thickness d1 in a second unit pixel region ‘b’ for realizing a color (e.g. G or yellow) of a second wavelength relatively shorter than the first wavelength, and etched at a third thickness d3 which may be relatively greater than the second thickness d2 in a third unit pixel region ‘c’ for realizing a color (e.g. B or cyan) of a third wavelength relatively shorter than the second wavelength. (2) first photodiode PD1 formed at a first depth d1 from the substrate SUB etched in the first unit pixel region ‘a’. (3) A second photodiode PD2 formed at a second depth d2′ which may be relatively greater than the first depth d1′ from the substrate SUB etched in the second unit pixel region ‘b’. (4) A third photodiode PD3 formed at a third depth d3′ which may be relatively greater than the second depth d2′ from the substrate SUB etched in the third unit pixel region ‘c.’ Here, ‘X’ may indicate the position of the substrate SUB before etching.
The distance from the rear surface of the substrate SUB to each of the bottoms of the first photodiode PD1, the second photodiode PD2, and the third photodiode PD3 may be substantially equal. In other words, the sum of the first thickness d1 and the first depth d1′, the sum of the second thickness d1 and the second depth d2′, and the sum of the third thickness d3 and the third depth d3′ may be substantially equal to each other.
The substrate SUB may be composed of a first conductive, such as P-type, relatively highly doped region P++ and a first conductive epi-grown region P-epi.
The first to third photodiodes PD1 to PD3 may each include a first impurity region P0 of the first conductive type extending from the surface of the substrate SUB to the bottom of the substrate SUB in each of the a, b, and c regions, and a second impurity region (n− region) of the second conductive type, such as N-type, adjoining the first impurity region P0 and extending from the first impurity region P0 to each of the formation depths d1′, d2′, and d3′ of the photodiodes PD1 to PD3.
A microlens ML may be formed on the top, overlapped with the photodiodes PD1 to PD3. According to the related art having the above-mentioned configuration, each color may be realized in the first to third unit pixel regions a, b, and c without respective color filters.
RGB, which may have three primary colors of light, may have different wavelengths. R may have a wavelength of about 0.55 μm to 0.6 μm, G may have a wavelength of about 0.45 μm to 0.55 μm, and B may have a wavelength of about 0.35 μm to 0.45 μm. Due to the relative differences in wavelength, the B color having a relatively short wavelength may have less depth of transmission through the silicon substrate SUB than the G color, and the G color may have less depth of transmission through the silicon substrate SUB than the R color.
Thus, the depth at which each color enters the substrate SUB and forms an electron hole pair may vary. By taking the depth of transmission varying with the wavelength differences between lights of respective colors into consideration, the n− region of the photodiode PD3 is made shallow in the unit pixel region c for realizing the B color having a short wavelength, the n− region of the photodiode PD2 is made deeper than ‘c’ in the unit pixel region b for realizing the G color having a longer wavelength than the B color, and the n− region of the photodiode PD1 is made deeper than ‘b’ in the unit pixel region a for realizing the R color having a longer wavelength than the G color.
White light is a mixture of all the wavelengths of light. The light of each wavelength may have different penetration depth. That is, light passes through the silicon substrate SUB and the wavelengths of RGB penetrate to different depths. The difference in penetration depth may be an optical characteristic, and therefore the penetration depth may not be adjusted as desired. As the wavelengths may penetrate to the penetration depth d1′ from the white light that has reached the surface of the substrate SUB of ‘a’, the wavelengths of the colors excluding the R color may be all extinguished and only the light having the wavelength of the R color may enter the first photodiode PD1 to generate photoelectrons.
As the wavelengths may penetrate to the penetration depth d3′ from the white light that may have reached the surface of the substrate SUB of ‘c’, R and G pass through the silicon substrate SUB and may be all extinguished, and only the light having the wavelength of the B color may enter the third photodiode PD3 to generate photoelectrons. The same applies to ‘b’.
According to the related art, colors may be realized without the use of color filters. In case of a product using a linear sensor, however, pixels may not be disposed like a general image sensor because they may have to be disposed transversely due to the characteristics of the linear sensor, and thus red, green, and blue may be disposed separately. Consequently, the longitudinal width of a chip may need to be maximized, thus relatively enlarging the overall chip size.